Heat the butter in a large frying pan over medium-high heat until foaming. Add the mushrooms and shallots and season with salt and pepper. Cook, stirring rarely, until the mushrooms are slightly browned and soft, about 5 minutes. Reduce the heat to medium, sprinkle in the flour, and stir to coat the mushrooms. Cook, stirring occasionally, until the flour is lightly toasted, about 3 minutes. Add the wine and stir with a wooden spoon, scraping up any browned bits from the bottom of the pan. Cook until the wine is completely reduced, about 30 seconds. Slowly pour in the broth or stock and whisk until smooth. Bring to a boil and cook, stirring occasionally, until the gravy has thickened and coats the back of a spoon, about 2 minutes. Remove the pan from the heat and stir in the cream. Taste and season with salt and pepper as needed.